1. Field
This disclosure relates to a method for epitaxial growth and an epitaxial layer structure using the method. More particularly, this disclosure relates to a method for epitaxial growth capable of securing stable optical and electrical characteristics by minimizing defects produced in a second epitaxial layer when growing the second epitaxial layer on a first epitaxial layer having defects formed therein, and an epitaxial layer structure using the method.
2. Description of the Related Art
Epitaxial growth refers to a process of forming a new monocrystalline layer on a monocrystalline substrate. The new monocrystalline layer formed by the epitaxial growth is referred to as an epitaxial layer. In the epitaxial growth, the monocrystalline substrate and the epitaxial layer may be formed of the same material (homoepitaxy) or of different materials (heteroepitaxy). In both cases, the lattice constant of the monocrystalline substrate needs to be identical or similar to that of the epitaxial layer.
When a material having a different lattice constant from that of a monocrystalline substrate is grown as an epitaxial layer to have a critical thickness or thicker, defects such as dislocation or micro-twin are inevitably produced in the corresponding epitaxial layer. Such a defect in the epitaxial layer is transferred to a thin film formed on the epitaxial layer, thereby deteriorating optical and electrical characteristics of the entire device. When the lattice constant of the monocrystalline substrate is identical to that of the epitaxial layer, subsequent epitaxial growth is also influenced by an inferior surface of the monocrystalline substrate, and therefore, a defect is induced in subsequent epitaxial growth.
Accordingly, studies have recently been conducted to remove defects produced in an epitaxial layer or to minimize the density of defects. Representative methods are as follows: a method of preventing defects from being transferred to a subsequent layer by laminating different semiconductor layers to change transfer directions of the defects (see FIG. 1); a method of preventing defects from being transferred by interposing an epitaxial layer at low temperature (see FIG. 2); a Pendeo epitaxy or epitaxial lateral overgrowth (ELOG) method of preventing defects from being partially transferred using a metal or other material and forming a high-quality epitaxial layer using lateral growth of a growth layer (see FIG. 3); and a method of preventing defects from being transferred by interposing a material layer having crystal structure and lattice constant similar to those of an epitaxial layer between a substrate and the epitaxial layer (see FIG. 4).
The methods of preventing transfer of defects according to related arts will be described in detail. First, the method of reducing the density of defects using a superlattice layer, shown in FIG. 1, is a method of reducing the Burgers vector of dislocation by applying strain or by changing the composition of a material. As long as the density of lattice defects is very low, it is known that the method is effective to a certain degree. However, if the density of the lattice defects is high or if a degree of the lattice defects is great, it is known that the effect and reproducibility of the method are degraded. The method of preventing transfer of defects using a superlattice layer has been disclosed in Erickson et al., J. Appl. Phys. 60, 1640 (1986), Russell et al., Appl. Phys. Lett. 49, 942 (1986), Umeno et al., Sol. Energ. Mat. Sol. C. 50, 203 (1998), and the like.
Next, the method of reducing lattice defects using a low-temperature buffer layer, shown in FIG. 2, is a method of preventing transfer of dislocations by interposing a thin buffer layer between a substrate and an epitaxial layer at low temperature. Although a clear mechanism has not been elucidated, it is known that a high-quality epitaxial layer is formed through a combination of some materials. Further, it is known that a kind of seed layer having a polycrystalline shape is formed through low-temperature growth, and a monocrystalline layer is then formed on the seed layer. Furthermore, it is known that the method is effective when a low-temperature GaN or AlN seed layer is formed on sapphire (Al2O3) and an epitaxial layer is then formed on the seed layer or when InSb is formed on a GaAs substrate. The method has been disclosed in U.S. Pat. No. 5,290,393 (Crystal growth method for gallium nitride-based compound semiconductor), and the like.
Next, the Pendeo epitaxy or ELOG method shown in FIG. 3 is a method of reducing lattice defects in a GaN light emitting device or the like. In the method, transfer of lattice defects is prevented using SiN, a metal, or the like, and growth of an epitaxial layer is induced in directions except for the direction perpendicular to a substrate, thereby minimizing lattice defects. However, an additional multistep process is required, and it is difficult to manufacture a device having uniform characteristics, because the device is divided into portions with no defect and portions with defects. The Pendeo epitaxy method has been disclosed in U.S. Pat. No. 6,265,289 (Methods of fabricating gallium nitride semiconductor layers by lateral growth from sidewalls into trenches, and gallium nitride semiconductor structure fabricated thereby), Lei et al., Appl. Phys. Lett. 59(8), 944 (1991), and the like.
Finally, the method of preventing lattice defects using a buffer layer having a similar lattice constant to an epitaxial layer, shown in FIG. 4, is a method of growing a GaAs or InP epitaxial layer on a silicon monocrystalline substrate without a defect, which has been developed by Motorola, Inc., and the like. In the method, a buffer layer having the crystal structure and lattice constant similar to those of an epitaxial layer is formed on a substrate, and the epitaxial layer is then grown on the buffer layer, thereby preventing lattice defects. In case of GaAs on Si, very excellent results have been reported by Motorola and the like. However, when the diameter of a substrate is great, cracks are produced in an epitaxial layer due to the difference of thermal expansion coefficient between respective layers. Therefore, reliability and reproductivity are degraded. Further, a separate deposition equipment is used to form a buffer layer, and therefore, efficiency is lowered. The method of preventing lattice defects using a buffer layer has been disclosed in Ishiwara et al., Jpn. J. Appl. Phys. 25, L139 (1986), Ishiwara et al., Jpn. J. Appl. Phys. 22, 1476 (1983), U.S. Patent Publication No. 2002/0030246 (Structure and method for fabricating semiconductor structures and devices not lattice matched to the substrate), and the like.
As described above, in the methods of preventing transfer of defects according to the related arts, defects in an epitaxial layer are minimized by forming a separate thin film such as a superlattice layer or buffer layer, or by using a separate deposition equipment. Therefore, processing is complicated and processing efficiency is lowered.
Meanwhile, the applicant has disclosed, in Korean Patent Registration No. 833897, a method in which quantum dots are formed on a first epitaxial layer, and defects are repaired by the quantum dots. However, complementary measures are required to increase defect repair efficiency.